Vacuum secondaries
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Joined: Apr 2019
Posts: 82
Likes: 10
From: Ohio
Vacuum secondaries
So i have a 69 corvette with:
-406 chevy small block
stock heads that came with the 406
-1970 lt1 cam and intake
-Holley 1850-4 with vacuum secondary, electric choke.
-stock m21 4 speed
-3.70 diff
-non a/c
-no power steering
-no power brakes
So my issue is that even with the smallest secondary spring, the secondaries wont open. I put a zip-tie on the assembly to check if they would and they will not. I have tried every spring and they wont open. My idle vacuum is around 7 in. Hg. Timing is 6 degrees under tdc.
I do have a shorter air cleaner on it, maybe 3/4 the size of stock because of clearance issues. Vacuum is good, headlights work great, vacuum advance is perfect, etc. Dad and I don't think the secondaries have ever worked, 91 is when Dad put the 406 in it with the 1850.
-406 chevy small block
stock heads that came with the 406
-1970 lt1 cam and intake
-Holley 1850-4 with vacuum secondary, electric choke.
-stock m21 4 speed
-3.70 diff
-non a/c
-no power steering
-no power brakes
So my issue is that even with the smallest secondary spring, the secondaries wont open. I put a zip-tie on the assembly to check if they would and they will not. I have tried every spring and they wont open. My idle vacuum is around 7 in. Hg. Timing is 6 degrees under tdc.
I do have a shorter air cleaner on it, maybe 3/4 the size of stock because of clearance issues. Vacuum is good, headlights work great, vacuum advance is perfect, etc. Dad and I don't think the secondaries have ever worked, 91 is when Dad put the 406 in it with the 1850.
Last edited by Mobifire; Sep 17, 2021 at 08:59 PM.
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Joined: May 2006
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You have the wrong timing, should be running at least 12 on a modified engine, which will partially account for the low vacuum reading.
Idle speed with that cam should be about 900 RPM.
Not too clear on height of filter, should be 3" height using a drop base air cleaner.
Secondaries not opening could be a torn diaphragm or a leaking seal between the main body and the secondary diaphragm housing.
Carburetor, 600 CFM, is small for that engine
Idle speed with that cam should be about 900 RPM.
Not too clear on height of filter, should be 3" height using a drop base air cleaner.
Secondaries not opening could be a torn diaphragm or a leaking seal between the main body and the secondary diaphragm housing.
Carburetor, 600 CFM, is small for that engine
Last edited by MelWff; Sep 17, 2021 at 11:39 PM.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,209
Likes: 9,347
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Have you tried openning them manually to make sure they arent seized shut?
Thread Starter
Advanced
Joined: Apr 2019
Posts: 82
Likes: 10
From: Ohio
You have the wrong timing, should be running at least 12 on a modified engine, which will partially account for the low vacuum reading.
Idle speed with that cam should be about 900 RPM.
Not too clear on height of filter, should be 3" height using a drop base air cleaner.
Secondaries not opening could be a torn diaphragm or a leaking seal between the main body and the secondary diaphragm housing.
Carburetor, 600 CFM, is small for that engine
Idle speed with that cam should be about 900 RPM.
Not too clear on height of filter, should be 3" height using a drop base air cleaner.
Secondaries not opening could be a torn diaphragm or a leaking seal between the main body and the secondary diaphragm housing.
Carburetor, 600 CFM, is small for that engine
edit: just checked, my filter is 2.7 inches tall.
Last edited by Mobifire; Sep 18, 2021 at 08:13 AM.
Race Director
Joined: May 2006
Posts: 18,729
Likes: 2,581
If you rebuilt it sounds like something went wrong.
Did you put a check ball in the vacuum secondary housing and if you did was it the wrong size check ball?
Did you put the small round gasket between the main body and the vacuum secondary housing?
Thread Starter
Advanced
Joined: Apr 2019
Posts: 82
Likes: 10
From: Ohio
Normal filter is 3", so why do you think it is too short.
If you rebuilt it sounds like something went wrong.
Did you put a check ball in the vacuum secondary housing and if you did was it the wrong size check ball?
Did you put the small round gasket between the main body and the vacuum secondary housing?
If you rebuilt it sounds like something went wrong.
Did you put a check ball in the vacuum secondary housing and if you did was it the wrong size check ball?
Did you put the small round gasket between the main body and the vacuum secondary housing?
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,209
Likes: 9,347
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Do you have the drop base filter?
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Thread Starter
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Joined: Apr 2019
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From: Ohio
Melting Slicks
Joined: Jun 2011
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So i have a 69 corvette with:
-406 chevy small block
stock heads that came with the 406
-1970 lt1 cam and intake
-Holley 1850-4 with vacuum secondary, electric choke.
-stock m21 4 speed
-3.70 diff
-non a/c
-no power steering
-no power brakes
So my issue is that even with the smallest secondary spring, the secondaries wont open. I put a zip-tie on the assembly to check if they would and they will not. I have tried every spring and they wont open. My idle vacuum is around 7 in. Hg. Timing is 6 degrees under tdc.
I do have a shorter air cleaner on it, maybe 3/4 the size of stock because of clearance issues. Vacuum is good, headlights work great, vacuum advance is perfect, etc. Dad and I don't think the secondaries have ever worked, 91 is when Dad put the 406 in it with the 1850.
-406 chevy small block
stock heads that came with the 406
-1970 lt1 cam and intake
-Holley 1850-4 with vacuum secondary, electric choke.
-stock m21 4 speed
-3.70 diff
-non a/c
-no power steering
-no power brakes
So my issue is that even with the smallest secondary spring, the secondaries wont open. I put a zip-tie on the assembly to check if they would and they will not. I have tried every spring and they wont open. My idle vacuum is around 7 in. Hg. Timing is 6 degrees under tdc.
I do have a shorter air cleaner on it, maybe 3/4 the size of stock because of clearance issues. Vacuum is good, headlights work great, vacuum advance is perfect, etc. Dad and I don't think the secondaries have ever worked, 91 is when Dad put the 406 in it with the 1850.
